Are other forms of BCI not permanent?
No, typically they're just sensors on a cranial harness.
Though, I kind think it being temporary would kind of be an upside, for the most part, since that would prevent scar tissue buildup on the brain, and other potential problems like that.
Yes, there's no real advantage to making it permanent other than convenience. However this convenience is imo massively outweighed by the very real possibility of meningitis. It's crazy that they got approval to transect the blood brain barrier for an implant. Other implants do this, but that risk is being weighed against things like potentially deadly seizures, not mild convenience.